We have developed standard laboratory measures of various aspects of inhibitory control. These include: (1) pain tolerance (subject-controlled modification of Tursky method); (2) 'strength of nervous system' (reaction time vs stimulus intensity); (3) augmentation-reduction (evoked response vs auditory stimulus intensity); (4) preception (reduction of autonomic response to noxious stimulation as a function of predictability); (5) habituation of the orienting reflex; and (6) Heyman's coefficient of inhibition (measured from effect of masking noise on auditory thresholds). We have also developed highly reliable psychometric indices of trait anxiety, neuroticism, 'fluidity', and control vs impulsivity. We propose now to study the interrelations of these measures on a large sample of adult same-sex twins.